Electrophotographic image-forming devices well known in the art form images on paper by first forming electrostatic latent images on photosensitive drums with laser beams reflected off a rotating polygon mirror and scanned in cycles over the photosensitive drums and by subsequently developing the electrostatic latent images with toner and transferring and fixing these toner images to the paper. A technology has been proposed for finely adjusting the magnification of the formed image in the main scanning direction in this type of image-forming device by controlling the rotating speed of the polygon mirror during the scanning process.